Available data indicate that genetic susceptibility plays a major role in the etiology of inflammatory bowel disease (IBD). In this study, phenotypes and genotypes related to immune function are studied in patients with IBD and their first degree relatives. Studies performed thus far suggest that several different genes within the HLA region on chromosome 6, along with several genetic loci on other chromosomes, are involved in the etiology of IBD. Active recruitment of IBD patients and their relatives will continue for ongoing genetic and immunologic studies.